1. Field of the Invention
The present invention relates to a benzoxazole derivative and a light-emitting element including the benzoxazole derivative. The present invention also relates to a light-emitting device, an electronic device, and a lighting device each including the light-emitting element.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively conducted. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not needed, and so on. Therefore, such a light-emitting element is regarded as being suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed.
Further, since such a light-emitting element can be formed to have a film shape, plane light emission can be easily obtained. Therefore, a large-area element capable of the plane light emission can be formed easily. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is very effective for use as a surface light source applicable to a lighting device and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. Then, these carriers (i.e., electrons and holes) are recombined, whereby the light-emitting organic compound is excited. The light-emitting organic compound returns to the ground state from the excited state, thereby emitting light. Note that the excited state of an organic compound can be a singlet excited state (S*) and a triplet excited state (T*), and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
At room temperature, a compound that is capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound) generally exhibits only luminescence from the singlet excited state (fluorescence), and does not luminesce from the triplet excited state (phosphorescence). Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% on the basis of S*:T*=1:3.
On the other hand, when a compound in which a triplet excited state is converted into luminescence (hereinafter, such a compound is referred to as a “phosphorescent compound”) is used, internal quantum efficiency can be theoretically 75% to 100%. In other words, emission efficiency which is 3 times to 4 times as much as that of a fluorescent compound can be obtained. For these reasons, a light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element.
Further, since there is a need for energy saving, development of a light-emitting element consuming less energy is needed.